A vinylidene fluoride based polymer is a crystalline polymer, and as having excellent mechanical strength, it is used in various articles. When a vinylidene fluoride based polymer is used as a binder solution for forming an electrode of a nonaqueous cell such as a lithium cell, the solubility of the polymer becomes an issue. For example, when a non-uniform solution in which a vinylidene fluoride based polymer is not completely dissolved is used as a binder for forming an electrode of a nonaqueous cell, powder electrode materials (powder materials such as an electrode active material and a conductive auxiliary agent) cannot be bound sufficiently and, furthermore, when a mixture of the powder electrode materials and a binder is applied to a charge collector, protrusions and depressions are formed to make it impossible to form a flat surface.
In cases of using a vinylidene fluoride based polymer as a binder for cells, those having an inherent viscosity of greater than 2.0 dl/g are excellent in binding capacity and the like (Patent Publication 2). An inherent viscosity is an alternate index of a molecular weight (degree of polymerization) of a polymer. The solubility of a polymer powder in an organic solvent decreases with increasing inherent viscosity to take time to dissolve.
Conventionally, a “good” solvent having adequate dissolving power with respect to a vinylidene fluoride based polymer is selected and the mixture is stirred with heating, as necessary, to prepare a uniform solution. However, since the vinylidene fluoride based polymer powders are aggregated during the stirring to form ball-like lumps that hinder the solvent to permeate into aggregate resin, it requires a long time to completely dissolve the powders to obtain a uniform solution. This tendency is stronger in a vinylidene fluoride based polymer with a higher degree of polymerization, which is useful as a binder for an electrode of a nonaqueous cell. For example, stirring for a long period of time of 2 days at 70° C. is required to prepare a solution with a concentration of 10% by weight by dissolving in N-methyl-2-pyrrolidone a vinylidene fluoride based polymer having an inherent viscosity (a logarithmic viscosity at 30° C. of a solution in which 4 g of resin is dissolved in 1 liter of N,N-dimethylformamide) of 2.1 dl/g. It may be a possible way to introduce a vinylidene fluoride based polymer by extremely small amounts to prevent formation of ball-like lumps, but this is not practical in view of workability and low efficiency.
The low solubility of vinylidene fluoride based polymers is attributed to dense surfaces of vinylidene fluoride based polymer powder particles that make it difficult for an organic solvent to permeate into inside the powder particles. Solution polymerization, emulsion polymerization, and suspension polymerization are known as methods for preparation of vinylidene fluoride based polymers. Solution polymerization is normally carried out at a polymerization temperature of 20° C. to 80° C. and a relatively low pressure (for example, 1.5 MPa or below), and is employed in production of relatively low-molecular-weight (for example, a number average molecular weight of 100,000 or below) vinylidene fluoride based polymers for use in paint. Emulsion polymerization is carried out using a chemically stable fluorine based emulsifier and a buffer to obtain a latex product with a small particle size of 0.1 μm to 1 μm, and the latex product is precipitated with an aggregating agent or the like and then subjected to granulation treatment and then formed into powder particles. Emulsion polymerization has disadvantages of complicated operations and high cost, because it requires use of an expensive emulsifier, separation of powder by aggregation/precipitation operations, removal of ionic substances contained in a aggregating agent, and so on. Suspension polymerization is a method that came into use since polymerization at a temperature of 30.1° C. (critical temperature of vinylidene fluoride monomer) or lower became possible as a result of development of an initiator capable of initiation at low temperatures. Vinylidene fluoride monomers, either alone or in combination with a copolymerizable monomer, are dispersed in water by use of a suspension agent to produce monomer droplets, and polymerization is developed in the presence of a polymerization initiator soluble in the droplets. The present Applicant proposed a suspension polymerization method comprising supplying a monomer dominantly comprising vinylidene fluoride first at a pressure below a critical pressure Pcr of vinylidene fluoride and then supplying at a pressure of at least Pcr (Patent Publication 1). In such a method of suspension polymerization with batch supply, an excess pressure increase in a polymerization system can be inhibited, and the resulting vinylidene fluoride based polymer has coloring resistance at a high-temperature, which is important for an appearance of a melt molded article.
However, vinylidene fluoride based polymers obtainable by the foregoing improved polymerization methods are used mainly to obtain a melt molded article having an inherent viscosity of 1.5 dl/g or below, and a problem regarding the solubility of the resulting polymers in an organic solvent and a solution to such a problem are not discussed.
Further, Patent Publication 1 discloses as a comparative example a polymerization example in which no additional monomer dominantly comprising vinylidene fluoride was added, but an inherent viscosity of the polymer obtained by this polymerization method was 1.1 dl/g. No vinylidene fluoride based polymer powder having an inherent viscosity of 2.0 dl/g or greater and excellent solubility was known in the past.    Patent Publication 1: WO 2006/061988    Patent Publication 2: Japanese patent No. 3703582